If you face an similar issue during your validation test of your failover cluster this could be also related to the Teredo IPv6 tunnel adapter. Teredo allow IPv6 communication to pass trough IPv4 NATs and IPv4 servers. In that case Teredo gives the same IPv6 address to his network interfaces which the validation tests of course flags as an error.

If you don’t use/require IPv6 you can find here detailed information’s how to disable Teredo :